Computers each of which is expected to be held by a user, such as cell phones, smartphones, and electronic book view devices (hereinafter, collectively referred to as “portable terminal devices”) are widely used these days. The portable terminal devices have an advantage of being usable by an operator while the operator is moving or walking, thereby enabling the operator to effectively use the time during moving or walking. However, there is a problem that, when the walking operator uses the portable terminal device, the device shake is caused by the walking movement of the operator and therefore the screen shake is also generated, making it difficult for the operator to watch the screen.
Techniques for improving the visibility of the screen when the screen is shaking have been proposed. For example, Patent Literature 1 describes an image capturing device which determines whether or not an object in captured image data is not shaking and stable and, in a case where the object is not stable, moves a zoom lens toward a wide end to enlarge the image data.
Moreover, applications actively using virtual reality have appeared as applications provided by computers. Some of the applications using virtual reality use the fact that the portable terminal device is held by the operator, detect the actual motion of the operator by the motion of the portable terminal device, and provide a field of view (display on a display unit) in a virtual world in accordance with the motion of the operator. Furthermore, other applications have also appeared, which estimate the motion of the operator from the motion of the portable terminal device and display a map for route guidance while moving the map in real time in accordance with the motion of the operator. In those applications, it is important how accurately the detection of the motion of the portable terminal device can be performed.
As a technique for detecting the motion of the portable terminal device, a technique is conventionally known which uses GPS, an acceleration sensor (for example, Patent Literature 2), a rotation sensor or the like. Moreover, a technique has appeared which detects the motion of the portable terminal device by an image captured by the portable terminal device. For example, Patent Literature 3 describes a technique which detects a face of a person using the portable terminal device and determines the displaying direction on a display unit to correspond to the axis of the face of that person. That is, the relative positional relationship between the portable terminal device and the axis of the user's face is determined by detection of the captured user's face, and is used as information for automatically changing the displaying direction on the display unit.
Patent Literature 1: JP 06-6666 A
Patent Literature 2: JP 2012-058847 A
Patent Literature 3: JP 2011-258191 A